Motion sensors are commonly fixed, but it is preferred, particularly for three component geophones, that one motion sensing element be vertical and the orientation of the other motion sensing elements known. In practice, geophones have spikes which are forced into the ground or allowed to lie on the ground at various angles. Measurements made with such geophones are less accurate than measurements made with geophones which are level with motion sensing elements oriented in a known direction. There has therefore been proposed a self-orienting geophone in which the motion sensing devices are held in a gimbal arrangement. This arrangement is somewhat complex and tends to be large, and it is believed that there is a need for a simpler self-orienting geophone with a different response to ground motion.